The present invention relates to an apparatus for processing an image signal having functions of halftone image reproduction in binary.
Recently, facsimile systems have become frequently used in daily business. There arises a demand for halftone reproduction of pictorial images in addition to back-and-white binary reproduction of documents or the like. However, halftone reproduction often has many restrictions from the viewpoints of recording apparatuses and transmission systems. For example, an apparatus for recording an image on a silver chloride film used in conventional photography or a heat-sensitive printing apparatus has good recording characteristics for halftone recording. However, an electrostatic copying machine or an ink jet printing apparatus has good characteristics for binary recording. On the other hand, regarding transmission systems, digital data transmission is taking over from analog data transmission. In this field, data compression schemes are used to perform high-speed data transmission. Under these conditions, a pseudo halftone display system having a binary recording apparatus is proposed, which is suitable for digital data transmission, thereby providing an optimum fascimile system.
An electronic halftone-dot generating method for a printed image in a newspaper or magazine and a dither method for digitizing or quantizing an image signal in accordance with a threshold matrix table are typical examples of a pseudo halftone display system. However, these conventional methods have poor resolution of a two-valued (binary) image such as a character or line. Therefore, the halftone portion or binary image portion cannot help but be sacrificed.
For example, according to the dither method, in a threshold window consisting of a threshold pattern of a plurality of different threshold levels, a multilevel input image signal is compared with the threshold levels in units of picture elements. When a given picture element level of the original image data exceeds the corresponding threshold, the picture element is set to "black". Otherwise, the picture element is set to "white". In this manner, each picture element is converted to binary data. When a 4.times.4 matrix window is used, 16 threshold levels can be set. Therefore, halftone display having 17 levels can be performed for the original image data. In this manner, according to the conventional dither method, black elements appear in each threshold window in a number corresponding to the original image data levels, so as to represent an average halftone mode. When the window size is small, the displayed image has good resolution. In this case, however, the number of halftone levels is decreased. On the other hand, if the window size is large, the number of halftone levels is increased. However, in this case, resolution is degraded. In addition to this disadvantage, the quality of a reproduced image of a binary original image portion is degraded in accordance with the conventional dither method, as compared with general binary processing.
Some of inventors of this application previously invented an apparatus and a method for converting halftone image signals into binary signals. The method is now patent pending in United States and other countries (U.S. patent Ser. No. 501,873, now U.S. Pat. No. 4,538,184; British patent publication No. 2,129,652, etc.) The method comprises defining a successively shifted scanning window with respect to the signal level of each picture element from which said video signal has been derived, totalizing the quantum numbers of the picture elements in the scanning window corresponding to the original screen dot, and reconstructing black picture elements corresponding to the total value of the quantum numbers faithfully to the area of the original dot. This method realizes binary data reproduction with no moire pattern, but it is not sufficient to obtain both of high resolution and multiple halftone levels.